The present invention relates generally to the data processing field, and, more particularly, relates to a technique for conserving power in operation of a hard disk drive.
In a hard disk drive, data signals are read from and written to a rotating disk by a head which is moved substantially radially with respect to the disk. FIG. 1 is a simplified block diagram that illustrates conventional circuitry which controls the positioning of a head in a disk drive. Signal conditioning and reading circuitry 10 receives an input signal from arm electronics, which are not shown. A signal output from the signal conditioning and reading circuitry 10 is provided to a head control circuit 12. The head control circuit 12 processes the signal received from the signal conditioning and reading circuitry 10 and outputs a control signal to an actuator drive circuit 14. The actuator drive circuit 14 provides a driving signal to a head actuator motor (not shown) which typically is a voice coil motor (VCM).
The head control circuit 12 also receives commands from a host system, which is not shown. The head control circuit 12 includes a processing capability 16, which may include one or more processors. Also included in the head control circuit 12 are one or more memory units, represented by memory block 18 in FIG. 1.
The head control circuit 12 responds to commands from the host by moving the head from a current position to a target position. Such operations are referred to as xe2x80x9cseeksxe2x80x9d, and are intended to bring the head into a stable position at a target track or cylinder.
According to conventional practices, commands from the host may be queued in the head control circuit 12, and may be selected from the queue for execution in accordance with a process intended to minimize the average access time for execution of commands. According to a known command queue ordering technique, a Shortest Access Time First (SATF) algorithm is employed. According to this algorithm, the command selected for execution from the command queue is that which can be started first. Estimated access times for commands are contained in a seek profile table stored in the memory 18. The estimated access times indicated by the seek profile table take two factors into account: (a) the time required to seek to and settle at the target track, and (b) the additional time required for the disk to rotate so that the target sector reaches the locus of the head. This additional rotational time is sometimes referred to as xe2x80x9cextra latencyxe2x80x9d.
When a command fails to be executed within the estimated access time indicated by the seek profile table, execution of the command must await an additional rotation of the disk. This phenomenon can degrade the performance of the disk drive, and is referred to as a xe2x80x9cmissxe2x80x9d. A command may fail to execute within the estimated access time indicated by the seek profile table as a result of factors such as variations in temperature, variations in head position, and external vibration.
The SATF algorithm has been modified according to an invention that is commonly assigned herewith and referred to as DEAT (Delta Expected Access Time). The DEAT SATF algorithm is disclosed in co-pending commonly-assigned patent application Ser. No. 09/638,253, filed Aug. 14, 2000 now U.S. Pat. No. 6,725,327. That patent application is incorporated herein by reference.
In selecting a command for execution from the command queue, the DEAT SATF algorithm considers not only the estimated access time indicated by the seek profile table, but also the probability that the command will be executed within the estimated time. The probability information may be based on actual experience in executing commands, as indicated by a probability table. A simplified example of a probability table provided according to the DEAT SATF algorithm is shown in FIG. 2. The probability table of FIG. 2 indicates execution experience for executed commands that have been categorized according to seek distance and estimated extra latency. The seek distance is measured in terms of cylinder groups, and the estimated extra latency is measured in terms of servo ID""s. In FIG. 2, four columns are shown, corresponding to four extra latency bands, and eleven rows are shown, corresponding to cylinder groups. For each category of commands, the number of misses and the number of xe2x80x9cmakesxe2x80x9d (occasions on which the command executed within the expected time) are recorded.
FIG. 2A is another example of a probability table used for the DEAT SATF algorithm, this time presenting data gathered from an actual implementation of the DEAT SATF algorithm. In contrast to FIG. 2, the table of FIG. 2A expresses the command execution experience in terms of percentages corresponding to miss rates. That is, the cell entries in FIG. 2A are derived by dividing the number of misses for each category by the total of makes and misses for the category. It will also be observed that the table of FIG. 2A has 240 categories, corresponding to 30 cylinder groups by eight extra latency bands. It should also be noted that in the data shown in FIG. 2A, for a given category or cell, a miss rate of zero is recorded when the total of makes and misses for that category is less than 10.
Power consumption may be a consideration in driving a head actuator. In many conventional disk drives, the head is driven to the target cylinder as rapidly as possible, without regard to the estimated extra latency for the command in question. This technique may use more power than is necessary to perform the command without incurring a miss. Power is used by the VCM only while accelerating or decelerating the actuator. To the extent that the head arrives at the target track prior to the target sector in the track rotating to the head locus, power is theoretically wasted.
One technique for reducing the amount of power consumed in driving the head actuator is to reduce (or xe2x80x9cclipxe2x80x9d) the maximum velocity of the actuator. This reduces the amount of acceleration and deceleration during seek operations, thereby reducing the amount of current drawn by the VCM. However, if the maximum seek velocity is reduced across the board, performance of the disk drive is adversely affected.
It could be contemplated to select a command for execution from the command queue based on considerations of power consumption, e.g., by giving priority to commands which require less power, but this would tend to adversely affect performance.
According to another power saving technique, two seek profile tables are provided, with one of the tables corresponding to a full power maximum seek velocity, and the second table corresponding to a lower power (lower maximum velocity). In operation with the two seek profile tables, the high power seek profile table and an associated probability table are employed in accordance with the DEAT SATF algorithm to select a command for execution from the command queue. Then, the corresponding entry in the lower power seek profile table is consulted to determine whether the selected command can also be expected to execute on time if constrained by the lower maximum velocity. If it is determined that the selected command will likely be executed on time even if executed at lower power, then the lower maximum velocity constraint is applied. Otherwise, the lower maximum velocity constraint is not applied.
This technique provides some savings in power without adversely affecting performance, but requires substantial memory space for storing the additional seek profile table. Furthermore, power savings are less than optimal, since the number of power levels is limited to the number of seek profile tables, and the number of seek profile tables that may be practically provided is limited by memory constraints.
It would be desirable to provide a technique for controlling head actuation that provides a greater reduction in power consumption without adversely affecting performance of the disk drive.
According to an aspect of the invention, a method of operating a disk drive is provided. The method according to this aspect of the invention includes receiving an access command and determining a category of the access command based on a seek distance and an estimated extra latency for the access command. The method further includes looking up a maximum seek velocity that corresponds to the determined category of the access command, and performing a seek operation in response to the access command and subject to the looked-up maximum seek velocity.
In at least one embodiment, the maximum seek velocity may be looked up in a table that provides different respective maximum seek velocities for different seek distance/estimated extra latency categories of access operations. The seek distance may be represented in terms of cylinder bands and the estimated latency may be represented in terms of servo ID bands. The access command may be a read command or a write command.
The method may further include selecting the access command from a command queue prior to the looking-up step. In at least one embodiment, the access command may be selected from the command queue in accordance with an SATF algorithm. The SATF algorithm may be of a type that takes into account an estimated probability that the access command can be performed within an estimated access time.
According to a second aspect of the invention, another method of operating a disk drive is provided. The method according to this aspect of the invention includes storing a plurality of different maximum seek velocities in a table, where the stored maximum seek velocities are indexed by parameters of access operations. The method further includes receiving an access command, determining parameters of the received access command, and selecting one of the stored maximum seek velocities from the table based on the determined parameters of the received access command. The method further includes performing a seek operation in response to the received access command and subject to the selected maximum seek velocity.
In at least one embodiment, the parameters of access operations and the determined parameters of the received access command may include seek distance and estimated extra latency.
According to a third aspect of the invention, a method of establishing respective maximum seek velocities for categories of access commands is provided. The method includes maintaining statistics for each of the categories of access commands, where the statistics indicate a number of occasions on which access commands of the respective category were performed within an estimated access time and the statistics also indicate a number of occasions on which access commands of the respective category failed to be performed within the estimated access time. The method further includes deriving a performance rate for each category of access commands from the statistics for the respective category, and performing an access operation associated with a specific one of the categories of access commands. The method further includes comparing the performance rate for the specific one of the categories of access commands with a threshold, and determining whether to increase, or determining whether to decrease, a respective maximum seek velocity for the specific one of the categories of the access commands based on a result of the comparing step.
According to a fourth aspect of the invention, a memory device storing a maximum seek velocity table is provided. The table contains a plurality of cells. Each of the cells hold a maximum seek velocity value for constraining execution of a category of access commands which corresponds to the respective cell. The cells are defined by a plurality of estimated extra latency bands and a plurality of seek distance bands.
The present invention adapts maximum seek velocities to respective categories of access commands based on actual experience in executing access commands of the respective category. In this way, a maximum seek velocity is adopted for each category at a level which is not likely to adversely affect performance of the disk drive. At the same time, optimum savings in power are achieved, since a different optimal maximum seek velocity is provided for each category of access commands.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of exemplary embodiments, the appended claims and the accompanying drawings.